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System Selection | Construction
Practices | Maintenance
The following recommendations are based on analysis of recent
failures. Issues bearing on the design, construction, and
maintenance of retaining walls, with particular emphasis on
proprietary Mechanically Stabilized Earth (MSE) walls, include
selection of the proper system for a given location, proper
construction practices, and proper maintenance and design
recommendations, which are important for long-term wall
performance.
System Selection
Responsibility The project engineer must ensure that the retaining wall system
selected for a given location is appropriate. MSE wall suppliers are only
responsible for the internal stability of their walls. The overall (global)
stability of an MSE wall system is the responsibility of the engineer who
selects this type of wall for inclusion into the plans.
Geometry Location geometry most often dictates the selection of a retaining wall
system. The Geotechnical Manual offers information regarding evaluation of
geometry and selection of various wall types. MSE walls are commonly used
on TxDOT projects; however, in many situations--especially cuts--MSE may not be
the most appropriate wall type. Often the additional excavation and shoring
required for installation of MSE walls in cut situations make them uneconomical
and difficult to construct. Sometimes MSE walls are selected because only a
geometric layout and a standard sheet are required in the plans (the final
detailed drawings are produced as shop drawings). This minimal design effort up
front makes MSE walls a popular choice among engineers with limited time and
resources. Although tied-back, soil nailed, drilled shaft and spread footing
walls all require considerably more design effort and time, they are preferable
in some cut situations.
The stability of each proposed retaining wall installation
must be evaluated. Usually this involves a simple review of the wall height,
site geometry, and soil borings. Walls with heights of 20 feet or less, situated
on level ground, with soils borings indicating Texas Cone Penetrometer (TCP)
blow counts in excess of 20 blows per foot should not require a detailed
analysis. Walls taller than 20 feet, situated on slopes, or on soils weaker than
20 blows per foot should be looked at more closely. In general, place walls on
any slope steeper than 4:1 only with a careful review of both short and
long-term stability. Of particular concern are walls placed on freshly cut
slopes, where the soil data may indicate high strengths at the excavation level.
Freshly exposed material will soften with time, and an assessment of long-term
strengths must be made when analyzing walls in this situation. Local districts
may want to modify these guidelines based on their experience with specific
projects and local conditions.
Soil Characteristics The Texas Cone Penetrometer is poorly correlated for very low soil strengths and
may yield overly conservative results. When evaluating stability of walls on
soils weaker than 20 blows per foot, it may be appropriate to conduct laboratory
or in-situ testing in addition to the TCP. Triaxial or direct shear laboratory
tests will generally yield more accurate soil strengths for this type of
analysis.
Engineers in the Geotechnical Branch of the Bridge Division are available to
assist with the determination of testing for specific situations and with the
slope stability analysis.
Construction Practices
Actual Soil Conditions Because soil borings are taken at discrete locations, it
is difficult to determine what soils conditions will be experienced over a wider
area. During construction of retaining walls, evaluate the proposed retaining
wall location and notify the project designers of potential problems. Of concern
are soils that are soft or wet, areas that are producing groundwater, and areas
that exhibit slope failures during excavation. Each of these indicates potential
stability problems and should be brought to the attention of the wall designer.
It may be necessary to remove and replace poor soils, install drains, or modify
the wall to address such field conditions.
Adherence to plans and specifications Assure adherence to plans and
specifications during construction, especially with respect to width of
reinforced volume, length of straps, and type of backfill used. A number of the
short and long-term retaining wall performance problems are the result of
contractor failure to adhere to specification and plan requirements.
Plumb MSE walls require particularly close attention to placement and compaction
of select fill. Monitor wall panels for verticality upon completion of the
backfilling of each panel. Initial panel batter should be modified as required
to achieve a plumb retaining wall. In many cases failure to evaluate panel
plumbness throughout construction has resulted in walls that are significantly
out of tolerance.
Weather Make close observation of the retaining wall and backfill after heavy
rainfall, particularly in areas with higher volumes of rainfall. Rain can soften
or loosen the compacted backfill, and any rain that seeps into the backfill can
increase pressures on the wall panels. Check the temporary surface cover for
cracks and quickly seal any cracks to prevent seepage into the backfill.
Base Backfill Backfill the excavated area in the base of retaining walls as
quickly as possible. Accumulation of groundwater or surface water in this area
will soften the soils and reduce the stability of the walls. Excavation at the
base of an existing wall for installation of storm sewer, roadway, of other
structure should not proceed without a determination of wall stability in the
excavated condition.
Filter Fabric Cohesionless select fill is subject to erosion and piping if
subjected to large quantities of water flowing into the wall. Filter fabric is
required at each panel joint and is designed to retain wall backfill while
allowing the water to pass. Gaps or voids in the filter fabric allow fill to
escape from behind the wall.
Sealing Sealing of coping joints prevents excessive quantities of water from
entering the top of the wall. The current RW(TRF) standard sheet requires all
coping joints be sealed. This item of work should be required in the field and
monitored for compliance.
Maintenance
Periodically inspect walls for evidence of backfill loss, loss of joint seals,
or movement. Reseal joints, particularly those that may allow surface water to
enter the wall backfill. If evidence of backfill loss is observed, backfill the
effected area with select fill if the area is accessible, or use flowable fill
if access is restricted. Water infiltration into voids in walls can cause
excessive pressures within the wall and result in displaced panels and wall
failures. Treat voided areas when they are small and manageable, as they will
always increase in size with time.
Design Recommendations MSE Walls have been the most common retaining wall type on TxDOT projects for
the past two decades. The advantages of MSE walls include their low cost, low
design effort, speed of construction, and attractive appearance. MSE walls will
continue to be used in large quantities on TxDOT projects in the coming years.
With this in mind, the Bridge Division recommends that the following be
considered on upcoming projects utilizing MSE walls:
Selection of backfill for MSE walls The 2004 Retaining Wall Standard
Specification (Item 423) lists four types of select backfill for MSE walls. Type
"B" is the default backfill for permanent MSE walls. It is a good quality
backfill, and will result in acceptable wall performance. Type A is a coarser,
higher quality material, exhibiting improved constructability and performance.
It is generally a more expensive backfill material, but should be considered for
projects where the enhanced performance would be desirable. With the
introduction of the new Type A material in the 2004 Standards Specifications,
it is no longer desirable for projects to include specific coarser backfill
gradations in project general notes. Type C backfill is used only on temporary
MSE walls, and is not appropriate for permanent walls. Type D backfill is a
free-draining, rock backfill. Type D is intended for use in MSE walls that are
subjected to inundation. Retaining walls subject to inundation should clearly
state that Type D backfill will be required below the 100-year water elevation
noted in the plans. Alternately, the entire wall volume may be specified as Type
D. For projects requiring Type A or D backfill in the MSE walls, either
the general notes or the wall layouts themselves should clearly designate the
required backfill type. If no backfill type is specified, the specification
reverts to Type B.
Consider increasing the minimum embedment of MSE walls from one foot to two feet
below finished grade On projects where a small amount of fill is to be placed
below the wall, the designer may want to specify a minimum embedment of two feet
below finished grade or natural ground, whichever is lower. The standard
embedment of MSE walls is currently required to be one foot unless otherwise
shown in the plans. Several Districts have begun requiring a minimum embed of
two feet. Two feet gives a greater margin of error against inaccurate surveys or
grading, and provides an additional measure of stability in soft soils. Projects
over hard ground, or requiring excavation into rock may want to retain the
one-foot embedment.
Discourage the placement of walls on slopes steeper than 4:1 Many soils in
Texas exhibit marginal slope stability at 3:1 or even 4:1. The additional load
of a wall on these slopes reduces their stability and may result in a failure.
If project requirements dictate walls on slopes (perched walls), a detailed
slope stability analysis should be performed, and measures should be taken to
assure wall stability.
Avoid the use of cement-stabilized backfill Although cement-stabilized backfill
is an option allowed in our standard specifications and is an easy short-term
solution, it compromises the long-term performance of the wall because it
reduces the walls flexibility and it does not allow drainage through the wall.
On projects where settlement is anticipated due to soft soil, a general note
should be added to the plans eliminating cement-stabilized backfill as an
option.
Retaining walls serve well, but there are some key points for successful wall
performance: the correct system must be chosen for each location, and proper
construction practices must be employed. Also, as described above, there are a
number of design and maintenance issues that are equally important. Feel free to
call Mark McClelland, P.E., at (512) 416-2226.
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